Circuit arrangement with power factor correction, and corresponding appliance

ABSTRACT

The circuit arrangement has a mains connection, a mains switch with two switching contacts, a demagnetization coil and a switch-mode power supply, which contains a driver circuit for producing a control voltage for the switching transistor of the switch-mode power supply. A first switching contact of the mains switch is arranged between the mains connection and the demagnetization coil in order to switch the demagnetization coil on and off, and the second switching contact is connected to a supply or control voltage for the driver circuit, in order to switch off the switch-mode power supply. In consequence, a power factor coil can be used between the mains connection and the switch-mode power supply for power factor correction, without the mains switch being loaded by the inductance of the power factor coil.

The present invention relates to a circuit arrangement having aswitch-mode power supply as claimed in the preamble clause of claim 1,and on an appliance having a corresponding circuit arrangement.

Switch-mode power supplies produce a heavily pulsed load on theelectrical power supply system, which leads to harmonic currents inelectrical power supply systems. This load occurs in the region of thevoltage maxima of the sinusoidal mains voltage, at which voltage maximaan energy-storage capacitor for the switch-mode power supply isrecharged. Appliances having a relatively high power consumption, suchas televisions with relatively large picture tubes, now therefore haveto comply with specific regulations relating to harmonic currents. Theharmonic load on the electrical power supply system caused by anappliance can in this case be stated as a so-called power factor.

Widely differing circuit concepts are known for improving the powerfactor, for example from DE-A-196 10 762, EP-A-0 700 145 and U.S. Pat.No. 5,986,898. These contain a second current path with a coil betweenthe mains rectifier and the primary winding of the transformer, with theinductance of this coil acting like a current pump, which is controlledby a switching transistor in the switch-mode power supply and inconsequence broadens the pulsed current flow of the switch-mode powersupply.

A further possible way to improve the power factor of a switch-modepower supply is to use a coil in the input area of the switch-mode powersupply. This coil is also referred to as a 50 Hz coil, a mains frequencycoil or a power factor coil. In order to avoid confusion with othercoils, the description therefore always uses the expression power factorcoil for this coil.

However, this power factor coil has the disadvantage that the currentflow in the power factor coil is interrupted suddenly when the mainsswitch is operated in order to switch off the appliance. The energywhich is stored in the coil must be dissipated, however. Since the openmains switch represents the highest impedance in the circuit, a veryhigh voltage is therefore developed across the switching contacts of themains switch, and causes an arc. This causes the mains switch to agemore quickly, and the switch represents a safety risk since, in theworst case, it becomes a potential fire source.

Other mains switches, in which the switching contacts open at a slowrate, are likewise at risk. In this case, although the voltage acrossthe contacts is not very high, an arc is nevertheless produced, however,and burns until the end of the corresponding power supply systemhalf-cycle. In this case as well, a large amount of energy is lost inthe switch, which leads to rapid aging.

The object of the present invention is to specify a circuit arrangementand a corresponding appliance of the type mentioned initially, whichhave a high reliability, combined with a low circuit complexity.

For a circuit arrangement, this object is achieved by the features ofclaims 1, 7 and 8, and for an appliance it is achieved by the featuresof claim 9. Advantageous developments of the invention are specified inthe dependent claims.

The circuit arrangement according to the invention has a mainsconnection, a mains switch with two switching contacts, ademagnetization coil and a switch-mode power supply, which comprises adriver circuit for producing a control voltage for the switchingtransistor of the switch-mode power supply. A first switching contact ofthe mains switch is arranged here between the mains connection and thedemagnetization coil in order to switch the demagnetization coil on andoff, and the second switching contact is connected to a supply orcontrol voltage for the driver circuit, in order to switch off thecontrol voltage for the switching transistor or in order to switch offthe switch-mode power supply.

This has the advantage that a power factor coil between the mainsconnection and the switch-mode power supply can be used for power factorcorrection, and that the mains switch is not loaded by the inductance ofthe power factor coil. Admittedly, this results in the switch-mode powersupply being connected to the mains even when it is switched off.However, this is not disadvantageous for the user and leads only to aslight power loss resulting from the starting circuit, when theswitch-mode power supply is switched off.

The circuit arrangement can be used in particular in appliances having apicture tube, for example televisions and computer monitors. This mainsswitch circuitry in this case ensures that the picture tube isdemagnetized whenever the appliance is switched on.

The invention will be explained in more detail in the following textusing, by way of example, an exemplary embodiment which is illustratedschematically in the figures, in which:

FIG. 1 shows a circuit arrangement having a mains connection, a powerfactor coil and a switch-mode power supply, as well as a mains switch,and

FIG. 2 shows the use of the arrangement in a chassis of a television.

FIG. 1 shows a switch-mode power supply I schematically, which has arectifier means, in this exemplary embodiment a bridge rectifier BR withfour diodes, an energy-storage capacitor C1 and transformer TR with aprimary winding W1, an auxiliary winding W2 arranged on the primary sideand secondary windings W3, W4 and W5 arranged on the secondary side. Theenergy-storage capacitor C1 is in this case arranged between the bridgerectifier BR and the primary winding W1. A switching transistor T1 isconnected in series with the primary winding W1 and is controlled by acontrol voltage DS for a driver circuit DC. A supply voltage VCC isprovided for operation of the switch-mode power supply via the auxiliarywinding W2, a diode D1 and capacitor C2, and is supplied to the drivercircuit DC.

The switch-mode power supply I in FIG. 1 is a part of a circuitarrangement which also has a mains connection NA, a demagnetization coil(which is not illustrated) and a mains switch S1 with two switchingcontacts 1, 2. The circuit arrangement is preferably integrated in anappliance, for example a television, which has a picture tube.

Picture tubes, which are used in televisions or computer monitors, needto be demagnetized from time to time in order to preserve the colorpurity of the picture tube. This is accomplished by means of ademagnetization coil, through which an AC voltage normally flows duringthe process of switching on the appliance. The 220 volt mains voltage isused as the AC voltage in this case and produces a high current surge atthe time when the appliance is switched on, and which then subsequentlydecays gradually. The decay is accomplished by means of a posistor,which is heated by the high current with its impedance in consequencebecoming high.

In order to regulate the switch-mode power supply, the driver circuit DCis supplied with a regulating signal RS which is derived from a supplyvoltage U4 on the secondary side, for example from the system voltage ina television, and is transmitted via an opto coupler or an isolatingtransformer (which is not illustrated) to the primary side of theswitch-mode power supply.

A so-called snubber network SN is arranged in parallel with the primarywinding W1 and is used to damp voltage spikes which occur during theprocess of switching off the switching transistor T1. The switch-modepower supply also contains a starting circuit AS, which supplies thedriver stage DC with a current for the starting phase of the switch-modepower supply after the appliance in which the circuit arrangement isintegrated has been switched on. The starting circuit AS is normally ahigh-value resistor chain, which produces a connection between thebridge rectifier BR and the capacitor C2. During operation, the supplyvoltage VCC is produced by the auxiliary winding W2, as well as by thediode D1 and the filter capacitor C2.

The switch-mode power supply illustrated in the figure preferablyoperates on the basis of the fly back converter principle, althoughother circuit principles are likewise possible. Fly back converters arepreferably used in entertainment electronics appliances, for example intelevisions and video recorders. In a fly back converter, energy is inthis case stored in the transformer TR during the phase in which theswitching transistor T1 is switched on, and is then subsequentlytransmitted to the windings W3-W5 on the secondary side, and to theauxiliary winding W2 on the primary side, in the phase during which theswitching transistor is switched off. Fly back converters are used bothas AC/DC converters and as DC/DC converters.

Switch-mode power supplies of this type have a low power factor, sincethe energy-storage capacitor C1 is recharged only in the region of thevoltage maxima and minima of the 50 Hz mains voltage, when the outputvoltage U2 of the bridge rectifier BR exceeds the voltage value acrossthe energy-storage capacitor C1. One simple possible way to improve thepower factor of a switch-mode power supply is to use a mains frequencycoil or power factor coil NS, which is arranged between the mainsconnection NA and the energy-storage capacitor C1. In this exemplaryembodiment, it is connected between the mains connection NA and themains rectifier BR.

The power factor coil NS results in the pulsed current flow which isused to recharge the energy-storage capacitor C1 being broadened andshifted in phase, since the inductance of this coil, for example 50 mH,means that the current flow through the coil rises only gradually, anddecays again in a damped manner. A power factor which complies with therequirements is achieved in this case by appropriate choice of theinductance value.

FIG. 1 likewise shows a mains switch S1, which has two switchingcontacts 1 and 2, as well as corresponding connections a and b for theswitching contact 1, and c and d for the switching contact 2. The mainsswitch S1 is in this case normally arranged on the front face of theappliance, such that it is easily accessible for a user. The connectionsc and d are in this case according to the invention connected to theconnections c′ and d′ for switching the driver circuit DC of theswitch-mode power supply on and off, and the connections a and b areconnected to the magnetization coil for switching the demagnetizationcoil on and off.

In consequence, the power factor coil NS is not arranged in the currentpath of the mains switch S1. The mains switch is therefore not loaded bythe power factor coil NS when the appliance is switched on and off. Atthe same time, this arrangement ensures that the demagnetization coil isswitched on, in order to demagnetize the picture tube, on each occasionwhen the appliance is switched on.

FIG. 2 shows the major components of the circuit arrangement, to theextent that they are integrated on a chassis of a television in anadvantageous refinement. On the input side, the arrangement has a filtersection with a mains filter NF, capacitors C3 and C4 and a resistor R1,which filter section is connected to the mains connection NA of thearrangement. This filter section prevents radiated interference from theappliance entering the mains system and suppresses any interferencewhich is already present on the mains system. On the output side, thefilter section is connected to the bridge rectifier BR, which providesthe rectified voltage U2 for operation of the switch-mode power supply.

Connections e and f are arranged in the connection between the bridgerectifier and the filter section, to which the power factor coil NS,which is not illustrated in FIG. 2, is connected. The power factor coilis arranged separately, so that the switch-mode power supply can be keptcompact and, in particular, such that an already existing switch-modepower supply which does not have power factor correction can be usedwithout any major design change. The arrangement of the power factorcoil NS upstream of the bridge rectifier BR means that it is located inthe current path of the sinusoidal AC voltage of the mains system.

The driver circuit DC and the transformer TR for the switch-mode powersupply are indicated only schematically in FIG. 2, to the extent that itis necessary for understanding of the invention. As already explainedwith reference to FIG. 1, a diode D1 and a capacitor C2 are arranged onthe auxiliary winding W2, which is arranged on the primary side, inorder to produce the operating voltage VCC for the driver circuit. Twoconnection points c′ and d′ are arranged on the chassis in theconnection between the capacitor C2 and the driver circuit DC, and areconnected to the connection points c and d of the mains switch S1.

A capacitor C6 with a capacitance of 1 μF is also advantageouslyconnected to ground between the connection c′ and the driver circuit DS,and is used to avoid switch bouncing of the switching contact 2 and toprovide filtering for the long supply line to the mains switch S1. Thecapacitance of this capacitor in this case also influences the number ofswitching cycles after which the switching transistor T1 is completelyswitched off.

A posistor PS and a capacitor C5 to which the demagnetization coil ES(which is not illustrated) is connected via connection points g and hare also arranged on the chassis. On the input side, the posistor PS isarranged in parallel with the capacitor C3. Connection points a′ and b′,which are connected to the connection points a and b of the mains switchS1, are in this case located in a supply. In consequence, thedemagnetization coil ES is switched on whenever the mains switch S1 isoperated in order to switch on the appliance. The posistor PS in thiscase ensures a large current surge at the moment of switching on, whichis reduced continuously to a low loss level by the heating of theposistor.

Whenever the appliance is switched, when the mains switch S1 is pressed,the switching contacts 1 and 2 are closed, so that the switch-mode powersupply can be started via the switching contact 2, since the drivercircuit DC is in this case supplied with the operating voltage VCC. Onbeing switched on, the switch-mode power supply thus behaves inprecisely the same way as when using the mains switch S1 in appliancesof a conventional type.

Whenever the appliance is switched off, when the mains switch S1 isopened, the switching contact 2 switches off the switching transistor T1in a short time, so that no more energy is transmitted in thetransformer TR to the second windings W2-W5. The switching voltage DSfor the switching transistor T1 is switched off directly or indirectlyby the driver circuit. The switching contact 2 is advantageouslyconnected to the connection points c′ and d′, as described withreference to FIG. 2, so that the driver circuit DC is disconnected fromthe supply voltage VCC during the switching-off process. However,another voltage of the driver circuit DC, for example a control voltagefor the driver circuit DC, can also be switched off in a correspondingmanner by the switching contact 2, or the switching contact 2 can beused to regulate the regulating signal RS at a predetermined voltagelevel, so that the switching transistor T1 is likewise switched offpermanently.

According to the invention, the mains connection NA is thus connected tothe bridge rectifier BR without being switched, without a mains switch.The switch-mode power supply I is thus always connected to theelectrical power supply system whenever the mains plug of the applianceis connected to the electrical power supply system. In consequence, thepower factor coil NS is not arranged in the current path of the mainsswitch S1, so that the life of the mains switch S1 is considerablyimproved.

Further refinements of the invention are within the scope of a personskilled in the art. The invention is not restricted to switch-mode powersupplies based on the flyback converter principle, as already explainedabove, and can also be used for other switch-mode power supply conceptswhen power factor correction is required. The invention can also be usedin appliances which do not have power factor correction. Thus, forexample, the connection points e and f can be short-circuited inappliances for countries which do not require power factor correction.In consequence, there is no need to modify the chassis in theseappliances for these countries.

1. Circuit arrangement having a mains connection, a mains switch with afirst and a second switching contact, a demagnetization coil and aswitch-mode power supply comprising a driver circuit, a transformer witha primary winding and an auxiliary winding for providing a supplyvoltage for the driver circuit, a switching transistor in series withthe primary winding, the driver circuit producing a control voltage forthe switching transistor, a rectifier means for rectifying a mainsvoltage, and an energy-storage capacitor coupled between the rectifiermeans sand the primary winding, the circuit arrangement comprisingfurther a power factor coil for power factor correction, which isarranged between the mains connection and said energy-storage capacitorwherein the first switching contact is arranged between the mainsconnection and the demagnetization coil for switching thedemagnetization coil on and off, and the second switching contact isarranged between the auxiliary winding and the driver circuit forswitching off the supply voltage, or is arranged for switching off acontrol voltage for the driver circuit in order to switch off theswitching transistor.
 2. The circuit arrangement as claimed in claim 1,wherein a diode and a second capacitor are coupled to a connection ofthe auxiliary winding in order to rectify and smooth said supplyvoltage, and in that the second switching contact is arranged betweenthe second capacitor and the driver circuit.
 3. The circuit arrangementas claimed in claim 1, wherein the power factor coil is arrangedupstream of the rectifier means.
 4. The circuit arrangement as claimedin claim 1, wherein the circuit arrangement comprises further a mainsfilter, a first parallel capacitor between the mains filter and themains connection and a second parallel capacitor between the mainsfilter and the rectifier means, that the demagnetization coil isarranged in parallel to the second parallel capacitor and in parallel tothe rectifier means, and that the connections (a, b) of the firstswitching contact are connected in series between the second parallelcapacitor and the demagnetization coil for switching the demagnetizationcoil on and off.
 5. The circuit arrangement as claimed in claim 4,wherein a posistor is arranged in series between the first switchingcontact and the demagnetization coil.
 6. Circuit arrangement having amains connection, a user accessible mains switch with a first and asecond mechanical switching contact, a demagnetization coil and aswitch-mode power supply comprising a driver circuit, a transformer witha primary winding and an auxiliary winding for providing a supplyvoltage for the driver circuit, and a switching transistor in serieswith the primary winding, the driver circuit producing a control voltagefor the switching transistor wherein the first switching contact isarranged between the mains connection and the demagnetization coil forswitching the demagnetization coil on and off, and the second switchingcontact is arranged between the auxiliary winding and the driver circuitfor switching off the supply voltage, or is arranged for switching off acontrol voltage for the driver circuit in order to switch off theswitching transistor.
 7. Appliance, having a circuit arrangement inaccordance with claim
 1. 8. The appliance as claimed in claim 7, whereinthe appliance comprises a picture tube, on which the demagnetizationcoil is mounted.